1. Field of the Invention
The present invention concerns an artificial kidney used in the extracorporeal treatment of blood, and in particular, in the treatment of blood by dialysis and by ultrafiltration. The present invention concerns more particularly an artificial kidney allowing the ultrafiltration of the blood to be controlled with great accuracy during a haemodialysis session.
The present invention also concerns a method for controlling and maintaining parity in the quantities of the dialysis liquid entering into and emerging from a dialysis liquid circuit of an artificial kidney.
2. Description of the Related Art
Artificial kidneys capable of controlling ultrafiltration of blood are known in the prior art. The article "Volumetrical Microcomputer Based Ultrafiltration Monitor For Hemodialysis" by J. M. P. Wokke, The International Journal of Artificial Organs, vol. 8 No. 1, 1985, describes an artificial kidney with a dialysis liquid circuit which comprises two flowmeters, one upline, the other downline from the haemodialyser. As a result of the pressure gradient obtained on either side of the semi-permeable membrane of the dialyser, a fraction of the liquid present in the blood passes by ultrafiltration through the membrane. The amount of flow of the dialysis liquid measured downline from the haemodialyser then exceeds the amount of flow measured upline from the haemodialyser. The difference in the measurement of the dialysis liquid flow downline and that of the dialysis liquid flow upline is considered to correspond to the ultrafiltration flow. In such a system, the errors inherent in the flowmeters are partly corrected by a calibration obtained by by-passing the haemodialyser and by letting the same flow of dialysis liquid circulate in the two flowmeters. When the apparatus is operating normally, that is to say, when ultrafiltration occurs, the amount of flow measured downline from the haemodialyser is no longer only the dialysis liquid flow but is the sum total of the dialysis liquid flow and the flow of the ultrafiltrate. The measurement of the downline flow then lacks accuracy because the correction effected by calibration is only valid for a specific value of flow equal to that during calibration. Further compounding errors resulting from the absence of calibration during ultrafiltration, are errors inherent in controlling the amount of blood passed through the dialyser during ultrafiltration.
U.S. Pat. No. 3,946,731 to Lichtenstein, discloses an artificial kidney comprising in series over the dialysis liquid circuit a reservoir for the dialysis liquid, an upline pump, an upline flowmeter, a haemodialyser, a downline flowmeter, and a downline pump. A device for collecting the ultrafiltrate is placed in parallel between the flowmeter and the downline pump and this device is intended to receive the excess of the dialysis liquid corresponding to the ultrafiltrate coming from the blood. In such an apparatus, the downline and upline pumps are operated synchronously in such a way that they deliver the same quantities of dialysis liquid at the inlet and outlet of the dialysis liquid circuit, save for the accuracy of the pumps. The apparatus and method disclosed in Lichenstein for controlling the ultrafiltration has inherent inaccuracies in flow monitoring because errors in the measurement of the ultrafiltration result from the inaccuracies of each of the upline and downline flowmeters as well as measurement errors in the parity of the quantity of dialysis liquid delivered by the pumps disposed upline and downline from the haemodialyser. Current market demands for haemodialysors require that the device be able to measure the quantities of the dialysis liquid at the input and output of the dialysis liquid circuit in the range of plus or minus 30 ml/hour. The ultrafiltration measurement apparatus and method taught by Lichenstein does not afford such accuracy over the control of the ultrafiltration.
It is therefore an object of the present invention to provide an artificial kidney which does not have the drawbacks of the prior art and is capable of controlling the ultrafiltration of blood during a haemodialysis session in a very accurate manner, or at least within plus or minus 30 ml/hr.
Another object of the present invention is to provide an artificial kidney comprising a portion of a dialysis liquid circuit wherein, whether there is or is not ultrafiltration of the dialysis liquid, the quantity of the entering dialysis liquid is kept equal to the quantity of the emerging liquid with very great accuracy.
It is a further object of the present invention to provide an artificial kidney and method of operating the same capable of compensating for the inherent errors in the sensors used for measuring the quantities of the circulating dialysis liquid.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.